Find the average length of the first names of customers whose last names are longer than 5 characters.
CREATE TABLE customers (customer_id varchar, firstname varchar, lastname varchar); INSERT INTO customers (customer_id, firstname, lastname) VALUES ('C001', 'John', 'Smith'),('C002', 'Jane', 'Johnson'),('C003', 'Doe', 'Brown');
```sql
SELECT AVG(LENGTH(firstname)) 
FROM customers 
WHERE LENGTH(lastname) > 5;
```
```sql
SELECT AVG(LENGTH(firstname)) 
FROM customers 
WHERE LENGTH(lastname) > 5;
```